Electro-optic modulators influence an optical mode and are understood to work by affecting the complex refractive index of a semiconductor waveguide and thereby affecting the transmission of light propagating therein. In such a device, a p-i-n diode normally is formed across the waveguide and injects charge carriers into the region in which the mode is propagating. The presence of significant numbers of charge carriers affects the local refractive index and thus the speed of light transmission. This effect can be used in switches, interferometers etc.
In SOI (Silicon On Insulator) optical devices the mode propagates in a silicon layer over an insulating layer supported on a substrate. The insulating layer can then act as a confinement layer for the charge carriers in the vertical direction. However, it has been found that significant numbers of charge carriers still escape laterally, and this adversely affects both the performance of the device and its reproducibility.